The present invention relates to a method for controlling nitrogen oxides levels in flue gases generated in catalyst regenerators in hydrocarbon catalytic cracking systems.
Modern hydrocarbon catalytic cracking systems use a moving bed or fluidized bed of a particulate catalyst. Catalytic cracking differs from hydrocracking in that it is carried out in the absence of externally supplied molecular hydrogen. The cracking catalyst is subjected to a continuous cyclic cracking reaction and catalyst regeneration procedure. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in a hydrocarbon cracking zone, or reactor, usually at a temperature of about 425.degree.-600.degree. C. The reactions of hydrocarbons in the hydrocarbon stream at this temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid cracked hydrocarbons and other vapors are separated from the coked catalyst and are withdrawn from the cracking zone. The coked catalyst is stripped of volatiles and cycled to a catalyst regeneration zone. In the catalyst regenerator, the coked catalyst is contacted with a gas, such as air, which contains a predetermined concentration of molecular oxygen to burn off a desired portion of the coke from the catalyst and simultaneously to heat the catalyst to a high temperature desired when the catalyst is again contacted with the hydrocarbon stream in the cracking zone. After regeneration, the catalyst is cycled to the cracking zone, where it is used to vaporize the hydrocarbons and to catalyze hydrocarbon cracking. The flue gas formed by combustion of coke in the catalyst regenerator is removed from the regenerator, and may be treated to remove particulates and carbon monoxide from it, after which it is normally passed into the atmosphere. Concern with control of pollutants in flue gas has resulted in a search for improved methods for controlling such pollutants. In the past, concern has centered on sulfur oxides and carbon monoxide. Nitrogen oxides have recently become a problem, at least partly because of the increased use of newer catalyst regeneration technology, e.g., use of platinum-containing combustion promoters to burn carbon monoxide.
The older, conventional type of FCC catalyst regeneration currently used in many FCC systems is an incomplete combustion mode. In such systems, referred to herein as "standard regeneration" systems, a substantial amount of coke carbon is left on regenerated catalyst passed from the FCC regeneration zone to the cracking zone. This may be, for example, a concentration of more than 0.2 weight percent carbon, usually about 0.25 to 0.45 weight percent carbon. The flue gas removed from an FCC regenerator operating in a standard regeneration mode is also characterized by a relatively high carbon monoxide/carbon dioxide concentration ratio. The atmosphere in much or all of the regeneration zone is, over-all, a reducing atmosphere because of the presence of substantial amounts of unburned coke carbon and carbon monoxide.
Prior to the introduction of zeolite-containing catalyst, there was little incentive to attempt to remove substantially all coke carbon from the catalyst, since even a fairly high carbon content has little adverse effect on the activity and selectivity of amorphous silica-alumina catalysts. Most of the FCC cracking catalysts now used, however, contain crystalline aluminosilicate zeolites, or molecular sieves. Zeolite-containing catalysts have usually been found to have relatively higher activity and selectivity when their coke carbon content after regeneration is relatively low. An incentive has thus arisen for attempting to reduce the coke content of regenerated FCC catalyst to a very low level, e.g., below 0.2 weight percent.
Several methods have been suggested for burning substantially all carbon monoxide to form carbon dioxide during regeneration, in order to avoid air pollution by the carbon monoxide, to recover heat and to prevent after-burning. Among the procedures suggested for use in obtaining complete carbon monoxide combustion in an FCC regenerator have been: increasing the amount of oxygen introduced into the regenerator relative to standard regeneration; and either substantially increasing the average operating temperature in the regenerator or including one or more carbon monoxide oxidation-promoting metals, such as platinum, in the system to promote carbon monoxide burning in the regenerator. Commercially, platinum has been employed in two ways: (a) on the catalyst itself, i.e., in low concentration on essentially all the particulate solids circulating in the cracking system; or (b) on a small amount of promoter particles, i.e., in high concentrations on only a very small fraction (less than 1%) of the particulate solids in the cracking system. Various solutions have also been suggested for the sometimes-related problem of after-burning of carbon monoxide. These solutions include addition of extraneous combustibles or use of water or heat-accepting solids to absorb the heat of combustion of carbon monoxide when the heat is released after the flue gas leaves the dense catalyst phase.
Complete combustion regeneration systems using a high temperature in the catalyst regenerator, rather than a promoter, to accomplish complete carbon monoxide combustion have been commercially employed. Much of the CO combustion takes place in a dilute catalyst phase in an after-burning mode, so that (1) more of the heat generated by carbon monoxide combustion is lost in the flue gas rather than absorbed in the catalyst, and (2) high temperatures are generated, with the possibility of a permanent adverse effect on the activity and selectivity of catalyst exposed to the dilute-phase flue gas.
Because of activity limitations, promoting metals, such as platinum, must be incorporated into particulate solids in relatively higher concentrations, e.g., 0.01 to 1 weight percent, when the promoted particles comprise a small fraction (e.g., less than 1%) of the total solids inventory in a cracking system.
When using carbon monoxide combustion-promoting metals associated with a small fraction of the total particulate solids inventory in a cracking system (including either promoted particulate catalyst or any other promoted solids in the system, or both), essentially complete carbon monoxide combustion has been obtained commercially. Low levels of coke on regenerated catalyst, another desirable result, have also been obtained. On the other hand, the amount of undesirable nitrogen oxides formed in the regenerator flue gas has substantially increased in catalyst regenerators using promoting metals contained on a small fraction of the circulating particulate solids. This has created a serious air pollution problem in disposing of the flue gas. Despite the NO.sub.x problems, use of combustion promoters comprising only a small fraction of the total solids inventory in a cracking system is still often preferable to use of a small amount of promoting metal on a large fraction of the solids. This is because of the flexibility permitted using a small amount of additive. E.g., use of the additive can be discontinued rapidly without removing a large portion of the catalyst inventory.
Several modes of addition of Group VIII noble metals and other carbon monoxide combustion-promoting metals to FCC systems have been suggested in the art. In U.S. Pat. No. 2,647,860 it is proposed to add 0.1-1 weight percent chromic oxide to an FCC catalyst to promote combustion of carbon monoxide to carbon dioxide and to prevent after-burning. U.S. Pat. No. 3,364,136 proposes to employ particles containing a small-pore (3-5 A.) molecular sieve with which is associated a transition metal from Groups IB, IIB, VIB, VIIB and VIII of the Periodic Table, or compounds thereof, such as a sulfide or oxide. Representative metals disclosed include chromium, nickel, iron, molybdenum, cobalt, platinum, palladium, copper and zinc. The metal-loaded, small-pore zeolite may be used in an FCC system in physical mixture with cracking catalysts containing larger-pore zeolites active for cracking, or the small-pore zeolite may be included in the same matrix with zeolites active for cracking. The small-pore, metal-loaded zeolites are suggested for the purpose of increasing the CO.sub.2 /CO ratio in the flue gas produced in the FCC generator. In U.S. Pat. No. 3,788,977, it is proposed to add uranium or platinum impregnated on an inorganic oxide such as alumina to an FCC system, either in physical mixture with FCC catalyst or incorporated into the same amorphous matrix as a zeolite used for cracking. Uranium or platinum is added for the purpose of producing gasoline fractions having high aromatics contents, and the effect on carbon monoxide combustion when using the additive is not discussed in the patent. In U.S. Pat. No. 3,808,121 it is proposed to supply large-size particles containing a carbon monoxide combustion-promoter metal in an FCC regenerator. The smaller-size catalyst particles are cycled between the FCC cracking reactor and the catalyst regenerator, while, because of their size, the larger promoter particles remain in the regenerator. Carbon monoxide oxidation promoters such as cobalt, copper, nickel, manganese, copper chromite, etc., impregnated on an iorganic oxide such as alumina are disclosed for use. Belgian patent publication No. 820,181 suggests using catalyst particles containing platinum, palladium, iridium, rhodium, osmium, ruthenium or rhenium or mixtures or compounds thereof to promote carbon monoxide oxidation in an FCC catalyst regenerator. An amount between a trace and 100 ppm of the active metal is added to FCC catalyst particles by incorporation during catalyst manufacture or by addition of a compound of the metal to the hydrocarbon feed to an FCC unit using the catalyst. The publication asserts that addition of the promoter metal increases coke and hydrogen formation during cracking.
U.S. Pat. Nos. 4,072,600 and 4,093,535 disclose the use of combustion-promoting metals in catalytic cracking systems in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.